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In a Lonely Cosmos, a Hunt for Worlds Like Ours

PEERING DEEPLY The primary mirror of the Kepler telescope. The crafts mission is to discover Earth-like planets in Earth-like places.Credit
Ball Aerospace

Someday it might be said that this was the beginning of the end of cosmic loneliness.

Presently perched on a Delta 2 rocket at Cape Canaveral is a one-ton spacecraft called Kepler. If all goes well, the rocket will lift off about 10:50 Friday evening on a journey that will eventually propel Kepler into orbit around the Sun. There the spacecraft’s mission will be to discover Earth-like planets in Earth-like places — that is to say, in the not-too-cold, not-too-hot, Goldilocks zones around stars where liquid water can exist.

The job, in short, is to find places where life as we know it is possible.

“It’s not E.T., but it’s E.T.’s home,” said William Borucki, an astronomer at NASA’s Ames Research Center at Moffett Field in California, who is the lead scientist on the project. Kepler, named after the German astronomer who in 1609 published laws of planetary motion that now bear his name, will look for tiny variations in starlight caused by planets passing in front of their stars. Dr. Borucki and his colleagues say that Kepler could find dozens of such planets — if they exist. The point is not to find any particular planet — hold off on the covered-wagon spaceships — but to find out just how rare planets like Earth are in the cosmos.

Kepler’s strategy is, in effect, to search for the shadows of planets. The core of the spacecraft, which carries a 55-inch-diameter telescope, is a 95-million-pixel digital camera. For three and a half years, the telescope will stare at the same patch of sky about 10 degrees, or 20 full moons, wide, in the constellations Cygnus and Lyra. It will read out the brightnesses of 100,000 stars every half-hour, looking for the telltale blips when a planet crosses in front of its star, a phenomenon known as a transit.

To detect something as small as the Earth, the measurements need to be done with a precision available only in space, away from the atmospheric turbulence that makes stars twinkle, and far from Earth so that our home world does not intrude on the view of shadow worlds in that patch of sky. It will take three or more years — until the end of Barack Obama’s current term in office — before astronomers know whether Kepler has found any distant Earths.

If Kepler finds the planets, Dr. Borucki explained, life could be common in the universe. The results will point the way for future missions aimed at getting pictures of what Carl Sagan, the late Cornell astronomer and science popularizer, called “pale blue dots” out in the universe, and the search for life and perhaps intelligence.

But the results will be profound either way. If Kepler doesn’t come through, that means Earth is really rare and we might be the only extant life in the universe and our loneliness is just beginning. “It would mean there might not be ‘Star Trek,’ ” Dr. Borucki said during a recent news conference.

The need, indeed even the possibility, of a planetary census is a recent development in cosmic history. It was only in 1995 that the first planet was detected orbiting another Sun-like star, by Michel Mayor and his colleagues at Geneva Observatory. In the years since then there has been a torrent of discoveries, 340 and counting, that has bewildered astronomers and captured the popular imagination.

“What exists is an incredibly random, chaotic, wild range of planets,” said Debra Fischer of San Francisco State University, also a veteran planet hunter who is not a member of the Kepler team. So far none of them qualify as prime real estate for life, and few of them reside in systems that resemble our own solar system. Many of the first planets discovered were so-called hot Jupiters, gas giants zipping around their stars in a few days in tight, blisteringly hot orbits.

Most of the planets have been found by what is called the wobble method, in which the presence of a planet is deduced by observing the to-and-fro gravitational tug it gives its star as it orbits. The closer a planet is to its star, the bigger the tug and the easier it is to detect.

The smallest exoplanet discovered is about three times as massive as the Earth. It is known as MOA-2007-BLG-192-L b, but astronomers don’t know yet whether its home star is real star or a failed star called a brown dwarf.

Last summer Dr. Mayor announced that his team had found three so-called warm super-Earths — roughly four, seven and nine times the mass of the Earth — orbiting within frying distance of a star known as HD 40307 in the constellation Pictor. Indeed, Dr. Mayor proclaimed that according to their data, about a third of all Sun-like stars host such super-Earths or super-Neptunes in tight orbits.

But all this is prelude. Astronomers agree that these planets are oddballs according to any reasonable theory of planet formation. But as Alan Boss of the Carnegie Institution of Washington pointed out, they are easy to detect by the wobble method. The fact that they are there suggests that there are many more modest-size planets to be found in larger, more habitable orbits.

The Kepler mission is a tribute to the perseverance of Dr. Borucki, who began proposing it to NASA in the 1980s, before any exoplanets had been discovered, and kept campaigning for it. “He had the true faith,” Dr. Boss said.

Many technical hurdles had be overcome before Kepler became practical. In particular it required very accurate and sensitive digital detectors, said James Fanson, of the Jet Propulsion Laboratory, Kepler’s project manager. As seen from outside the solar system, the Earth blocks only about 0.008 percent of the Sun’s light when it passes in front, or “transits.” Kepler has been built to detect changes in brightness as small as 0.002 percent, equivalent to a flea crawling across a car headlight.

By measuring the diminution of a star’s light during an exoplanet transit, astronomers in principle will be able to determine the size of the exoplanet. From the intervals between eclipses, astronomers will be able to determine its orbit.. By combining this with other data, from, say, wobble measurements, they will be able to zero in on important properties like mass and density.

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However, natural variations in the star’s output, caused by something like starspots, could interfere with the data and obscure the signals from small planets. That is a problem, Dr. Fanson said, with the Corot satellite, which was launched by the European Space Agency at the end of 2006 and also carries a telescope and camera to look for small changes in starlight. To weed out the noisy stars, Kepler will keep track of 170,000 stars for the first year and then narrow its attention to a mere 100,000.

Corot, which stands for convection, rotation and planetary transits, is smaller than Kepler and is designed to investigate the structure of stars by detecting vibrations and tremors within them that cause them to periodically brighten and fade. Corot, which Dr. Borucki called “a complementary mission,” also looks at a given patch of stars for only a few months and so would miss the successive transits of an Earth-like planet, which, to be habitable, would have to take about a year to orbit a Sun-like star.

Not all 100,000 stars in the field of Kepler’s view would have their planetary systems oriented to provide eclipses from our particular point of view, of course. Dr. Borucki and his colleagues estimate that for an Earth-like star in its habitable zone, the stars would align to produce a blot out in half of 1 percent of cases, yielding a few dozen to a few hundred new Earths out there. For planets that are closer in, however, the odds rise to about 10 percent, so there are ample reasons to expect a bumper crop of new planets.

Dr. Borucki said the astronomers had set the goal of observing at least three such transits, to confirm the period and rule out interference from starspots, and then obtaining backup observations from other telescopes — of wobble measurements, say — before announcing they have found a planet.

“When we make a discovery we want it to be bulletproof,” Dr. Borucki said.

That means that the first planets to be discovered and announced will be the biggest planets with the shortest orbits, the so-called hot Jupiters. Four stars with such planets are in the search area and thus will be an early test of Kepler’s acuity.

“In the first six months, hot Jupiters are going to roll off the Kepler assembly line,” Dr. Fischer said, adding, “These are bizarre planets, we don’t understand how they form.”

The hardest and most exciting part of the mission, detecting bona fide Earths, will also take the longest. Such a planet should take about a year to circle a Sun-like star, producing only one blip a year in its starlight. So it would take more than three years to produce the requisite three blips and subsequent confirmation by ground-based telescopes before the epochal discovery is announced.

Dr. Borucki said, “We’re not going to be able to tell you very quickly.”

But they will eventually tell us.

Dr. Boss, a high-ranking member of the Kepler science team, said: “It really is going to count many Earths. About four years from now we will have a really good estimate of how many Earths there are.”

If the history of exoplanet astronomy is any guide, there are likely to be surprises that geologists had not imagined — water worlds, for example. And then, if all keeps going well, it will be time to confront the next series of questions: whether anywhere else in this galaxy the dust that once spewed from stars has come alive and conscious.

“In my 25 years of working with NASA this is the most exciting mission I’ve worked on, said Dr. Fanson, who will step down as project manager after the launching. “We are going to be able to answer for the first time a question that has been pondered since the time of the ancient Greeks. Are there other worlds like ours? The question has come down to us from 100 generations. We get to answer it. I find that tremendously exciting.”

When a reporter departed from journalistic objectivity to venture a hope that other Earths are out there, Dr. Fanson happily joined in. “I hope the answer is yes,” he said. “I hope the universe is teeming with planets like Earth.”

A version of this article appears in print on , on Page D1 of the New York edition with the headline: In a Lonely Cosmos, a Hunt for Worlds Like Ours. Order Reprints|Today's Paper|Subscribe